A few years ago I wrote software that would periodically send and receive encrypted mail using the JavaMail API. We used Java 1.4 and we had to use the Bouncy Castle cryptography library to encrypt and decrypt the mail.
So yes, it is possible. Unfortunately it's already three years ago or so that I did this so I forgot all the details about how to do it. The Bouncy Castle website may contain some examples. [ January 23, 2007: Message edited by: Jesper Young ]